Colloidal metal conjugates can be used in a variety of applications, including assays, for example, immunoassays. In such applications, the colloidal metal conjugate can be a metal particle bound to an antibody which is capable of binding with a specific antigen. The metal particle can allow the binding to be visualized in an assay by optical or electronic means, including, for example, visual or spectrophotometric observation of color imparted by the size, composition and shape of the metal particle.